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Skills · skills/mcmaster-parts/SKILL.md · Markdown

McMaster-Carr parts

Bought hardware is a real part: a part number, its manufacturer's model, its specification. McMaster-Carr publishes a STEP model and a full specification table (material, finish, strength class, thread, dimensions and tolerances, load and speed ratings, temperature range, compliance) for almost everything it sells. Use them.

The tools: cadagent.mcmaster (Python) and cad-agent mcmaster (CLI). They read one product page at a time in the system Chrome (Playwright), with a pause between pages. That is the same traffic as an engineer choosing parts, and it must stay that way.

Rules

Process

  1. Pick the part like an engineer. Open the category page (e.g. https://www.mcmaster.com/products/ball-bearings/), filter it (System of Measurement → Metric, then size, material, thread) and read the table. Choose by function (load, fit, material, strength class) and by the SKUs you already use. Only then open the product page https://www.mcmaster.com/<PN>/.

  2. Download and normalise.

    cad-agent mcmaster 91290A242 --kind screw --out references/vendor/mcmaster \
    --catalog catalog/hardware.json --key screw_m5x20

    This writes OUT/<PN>/<PN>.step (the download, kept only if it is a STEP file), <PN>.page.json (title, price, pack quantity, every spec row, option choices), <PN>.norm.step, <PN>.norm.json (axis found, how it was seated, bbox, volume, mass) and an entry in SOURCE.txt. Kinds and their frames (cadagent.mcmaster.FRAMES) all put the axis on +Z, through the origin (the cylinders' axis line, not the centre of mass); which end goes down is decided from the part's shape, not from how the vendor modelled it:

    • bearing, washer, nut, insert, standoff, spacer, pin, grommet, rod: the larger end (flange, collar, head) at z = 0 and the part on +z; when both ends are the same size, the heavier end down (info["end_down"] says which rule decided).
    • screw: head on +z, shank towards -z. z = 0 is the underside of the head for socket, button, pan, hex and flanged heads, and the top face for a countersunk (flat) head (info["head"]). A part with no head wider than its shank (set screw, stud) is refused: use pin or rod.
    • shoulder_screw: the underside of the head at z = 0, head on +z, shoulder and thread towards -z.
    • rod_end: the ball centre at the origin, ball bore along +Y, threaded shank along -Z (without a modelled ball, the middle of the bore).

    A part with variants (a rod end's thread direction, a choice of finish) offers its CAD only after the choice. Give the option text after the part numbers, one --choose per option, prefixed with the part it is for:

    cad-agent mcmaster 59935K53 91290A242 --choose "59935K53:Right Hand" --out references/vendor/mcmaster

    (the PN: prefix may be left out when there is one part number). In Python: Session.fetch(pn, out, choose=["Right Hand"]). The page data records what was offered and what was chosen.

    Playwright: pip install playwright into the environment that runs it, plus Google Chrome (then nothing else is needed; without Chrome the command stops with an install hint).

  3. Record the catalogue entry. catalog_entry() / --catalog gives: mcmaster (PN), description, url, price_usd, pack_qty, dims_mm, material, mass_g with mass_basis (McMaster's listed weight when the page gives one, otherwise CAD volume x density, which it says), cad (the normalised STEP path), frame, and specs.raw (label → value) with specs.parsed (mm, g, N, MPa, deg C). Add what only the project knows: used_for, the quantity, and notes (stack and engagement arithmetic, CAD changes needed). Fields you add are kept when the entry is refreshed.

  4. Check the model. Compare bbox_mm with the page's dimensions (a bearing's OD and width, a screw's length below the head, a rod end's centre-to-end length). A mismatch means the wrong frame or the wrong part.

  5. Place it in the assembly. Import <PN>.norm.step and move it to the seat: for a screw, origin on the seat face and +Z out of the hole; for a bearing, the bottom face on the shoulder. Run the clash and tool-access checks (lesson 04) with the real heads, nuts and washers in place, not envelopes.

Python

import json

from cadagent import mcmaster as mc

with mc.Session(delay_s=3) as s:                    # one browser, one page at a time
    step = s.fetch("5972K82", "references/vendor/mcmaster/5972K82")
norm, info = mc.normalise(step, "bearing", material="Steel")
page = json.loads((step.parent / "5972K82.page.json").read_text())
mc.update_catalog("catalog/hardware.json", "bearing_6001",
                  mc.catalog_entry(page, info, cad=str(norm)) | {"used_for": "knee", "qty": 4})